This invention relates to a process for the production of crude shale oil suitable for use as a feed stock for refineries producing motor fuel and other hydrocarbon products, and particularly it relates to such a process wherein crushed oil shale is retorted in conjunction with small quantities of coal by the indirect application of heat obtained when the residual char is burned. An apparatus for the process is also disclosed.
It is well known that there is an increasing shortage of crude oil in the United States as older oil fields become depleted and new fields are difficult to locate and expensive to develop. It is also well known that immense reserves of petroleum are found in the shale of several western states. Research by the U.S. Bureau of Mines and others have proven that the oil produced from retorting this shale is suitable, after minor treatment, for refining into normal petroleum products. The techniques for mining this shale in an efficient manner with little or no adverse effect on the environment have been developed by the Bureau of Mines. Likewise techniques for disposal of spent shale in an ecologically sound manner have been published.
However until this time no economic process for retorting the shale has been invented, and as a result, no commercial shale oil producing facility has ever been constructed and operated in the United States. Several small facilities have been operated in foreign countries, especially in areas without alternative supplies of petroleum, however, the large labor requirement of the processes used made them impractical for the United States.
The largest United States deposit of oil shale is known as the Green River deposit and occurs in Colorado, Utah and Wyoming. This shale is a magnesium marlstone with a finely laminated structure wherein the organic material, called kerogen, and the inorganic material are intimately mixed. The shale contains from about 60 weight percent mineral matter to about 92 weight percent and this distinguishes it from coal which contains only minor quantities of minerals. The organic portion is of such a structure that it is virtually insoluble in common organic solvents and therefore can only be released by distructive distillation. The mineral portion is reported to consist largely of dolomite, calcite, feldspars, quartz and illite clay. Trona, Nahcolite, and Dawsonite are reported to occur in or interbedded with some deposits. The inorganic particles are very small with a mean size of about 6 microns. In its natural state the shale is a strong, impervious rock consisting of the finely divided inorganic particles cemented together with the kerogen as the binder.
Upon the application of heat the kerogen decomposes into useful products. At temperatures on the order of 900.degree.F a disproportionation of carbon and hydrogen structures occurs whereby the solid high molecular weight kerogen, which has a carbon to hydrogen ratio of about 7.8, is converted into a liquid oil with a carbon to hydrogen ratio of about 7.2. The yield of this oil is in the range of 60-70 percent of the organic matter in the raw shale. An additional 7 to 10 percent is converted into light gases and about 20 to 25 percent into a carbon rich residue retained on the inorganic material. This residue is called char. The oil thus produced normally has an API gravity of about 17 to 20 and a pour point of 70.degree. to 90.degree.F. At the temperatures of retorting, the oil is evaporated out of the shale.
Prior work attempting to develop economic processes for utilizing oil shale has followed several different paths. First is in situ retorting where the shale is burned in place and the heat produced decomposes the surrounding shale. This has been largely unsuccessful because of the impermeability of the shale which prevents movement of gases including both the air required for combustion and the product vapors.
Second is direct combustion retorting where crushed shale is heated by combustion occuring in the retort by burning injected fuels and/or the residual carbon remaining in the retorted shale. Commonly this is done in a vertical vessel where fresh shale is fed continuously or batchwise into the top and spent shale is removed from the bottom. Air for combustion is forced into the bottom section where combustion occurs. The hot gases pass up through the shale causing the kerogen to decompose. The product is removed as a vapor out the top and condensed. Equally common are down draft designs where the shale is fed upward and combustion occurs at the top with product removed at the bottom. These designs have the advantage of good heat efficiency but the disadvantage that the product is diluted with the combustion gases making recovery especially of light hydrocarbon gases difficult. Also since the shale contains large amounts of calcite and dolomite which decompose endothermally at 1050.degree.-1100.degree.F, temperature control in the combustion zone is very critical and very difficult. Also the best American shales tend to cake and fuse under extended combustion condition making continuous discharge of spent shale difficult if not impossible.
A third type of retorting process uses hot gas to heat the shale to the temperature required for destructive distillation. In some variations, flammable gases or liquid fuels are burned either within the retort or outside it and the gases produced by the combustion are passed through the bed of shale. The shale oil produced is swept out with the gases and condensed. This process has the advantage of good temperature control and the disadvantages of dilution of the product with the undesirable products of combustion, and high fuel requirements.
Most of the directly heated retorting systems have attractive potentials, but although they have been known for many years, they have never been commercialized because the design of the necessary equipment of a size to be profitable has eluded the designers and engineers.
Another system of retorting involves the indirect heating of the shale using ceramic balls to convey the heat. The spent shale is burned in a separate vessel to supply the heat to raise the temperature of the balls to such a point that when they are mixed with the shale in a retort, the shale is heated to retorting temperatures. The spent shale and the balls are then separated and the balls recycled. This process has the advantage that the light gases produced during retorting are not diluted with the products of combustion. The disadvantages are largely mechanical. Many other processes have been reported.
In order for a shale retorting process to be of commercial importance, it should meet all of the following criteria:
1. It must be mechanically sound. That is it must be possible to design and construct at a reasonable cost equipment to carry out the process on a very large scale and such equipment must be highly reliable and trouble free.
2. The process must produce a high yield of oil and saleable high BTU gas.
3. The process must be sufficiently efficient in regard to heat consumption that little fuel is required other than the char which results from retorting.
4. The process must use a relatively small amount of water since the oil shale occurs in arid areas where little water is available.
Until now no process has met this combination of requirements.